refactor(linear_algebra/bilinear_form): split off matrix part (#12435)

The bilinear form file is way too large. The part that concerns matrices is not depended on by the general theory, and belongs in its own file.

Author: mcdoll

src/algebra/lie/skew_adjoint.lean

@@ -4,7 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Oliver Nash
 -/
 import algebra.lie.matrix
-import linear_algebra.bilinear_form
+import linear_algebra.matrix.bilinear_form
 
 /-!
 # Lie algebras of skew-adjoint endomorphisms of a bilinear form